Unable to have more then four soul shards

I have been unable to have more then 4 soul shards for a vary long time. It didn't bother me because i wasn't leveling at the time, now i am and it dose.

I have enough room in my inventory to get more.

I have been using Soul Drain on level appropriate mobs.

When i kill a mob using Drain Soul I am prompted with a message saying "You can't carry any more of those items"

I did delete my warlocks wtf file and it didn't help.

So what can i do to make this problem go away?